doc: use HTTPS for man.openbsd.org link in go1.7.html

Change-Id: I88855dfa3166e90386c90bf6220be0596dab3c0d
Reviewed-on: https://go-review.googlesource.com/118175Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.7.html

@@ -89,7 +89,7 @@ POWER5 architecture.
 </p>
 
 <p>
-The OpenBSD port now requires OpenBSD 5.6 or later, for access to the <a href="http://man.openbsd.org/getentropy.2"><i>getentropy</i>(2)</a> system call.
+The OpenBSD port now requires OpenBSD 5.6 or later, for access to the <a href="https://man.openbsd.org/getentropy.2"><i>getentropy</i>(2)</a> system call.
 </p>
 
 <h3 id="known_issues">Known Issues</h3>